I also think it most likely that Irene pushed Clare and that the portrayal of her mental state was building up to her snapping in rage like that. Furthermore, I think that she thinks she did based on her reactions after the event and that she is surprised when she rejoins the group afterwards that they aren't accusatory towards her.
While I did read some ambiguity into it, the thought that her husband did it wasn't even one I had considered seriously. I also don't think suicide fit well with Clare's character. Accident seemed too much of a non-ending.
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